The present invention relates to a complex metallographic structured or multi-phase steel.
With ever-increasing pressure on the automotive and other industries for energy savings and emission reduction while improving product performance and cost competitiveness, more parts such as automotive parts are being manufactured using high strength steel. Some high strength steels enable use of thinner sheet to reduce the product weight, which improves vehicle fuel efficiency. Further, it is desired to improve vehicle durability, crashworthiness, intrusion resistance and impact performance to protect a driver and passengers upon collision.
Certain industries, including the automotive industry, are utilizing advanced high strength steel, or AHSS, including dual phase steels and transformation induced plasticity, or TRIP, steels. AHSS steels may meet certain strength and weight targets while using existing manufacturing infrastructure. These steels appear promising for applications requiring high press-forming and draw-forming properties to form parts with complex shapes.
However, problems related to the stamping, forming and drawing of dual phase steel and TRIP (transformation induced plasticity) steel are well known, and significant hurdles exist for successful implementation using the existing manufacturing infrastructure. Prior advanced high strength steels exhibited shear fracture, edge fracture, and edge cracking during the stamping or forming of a variety of parts. These fractures may occur when stretching steel over a radius or when stretching an edge of the part. These fractures have occurred on the radii of part geometries at strains well below the expected forming limit of the steel sheet. Further, fractures in prior AHSS occur in the absence of any appreciable thinning or necking normally associated with this type of fracture or cracking in conventional steels. In this way, these fractures have not been predictable using the existing norms of conventional steels, limiting design flexibility and increasing manufacturing uncertainty for prior AHSS.
Moreover, high concentrations of some alloy elements, such as carbon (C), silicon (Si) and aluminum (Al) present in TRIP steels deteriorate the surface quality and weldability of the steel. In particular, difficulty in welding TRIP steels has become a significant challenge for the steel in the automotive industry, and therefore further limits automotive applications of this type of steel.
The above limitations have recently been recognized by some producers and users of the dual phase steel and TRIP steel sheet. Consequently, investigations have been initiated to understand and characterize the fracture mechanisms of high strength dual phase and TRIP steels. There remains a need for a new type of advanced high strength steels to reduce the occurrence of the shear fracture during stamping and forming while improving the structural performance of automotive parts.
U.S. Pat. No. 6,641,931 to Claessens, et al. provides a method of producing a cold rolled metal coated multi-phase steel, characterized by a tensile strength of at least 500 MPa, a yield ratio lower than 0.65 in skinned conditions, and lower than 0.60 in un-skinned conditions. The hot metal coated steel product having a steel composition, by weight, of not more than 1.5% manganese, 0.2 to 0.5% chromium and 0.1 to 0.25% molybdenum, undergoes a thermal treatment in the hot dip metal coating line defined by a soaking temperature between Ac1 and Ac3, a quenching at a primary cooling speed higher than 25° C./s and a secondary cooling speed higher than 4° C./s.
The steels produced using U.S. Pat. No. 6,641,931 method have a low yield ratio, or lower yield strength, which suggests to one skilled in the art that the steel is actually one type of dual phase steel. The method described in this patent requires a strict two steps of cooling rate control, which is difficult to carry out during commercial production in a steel mill, and thus can restrict the commercial application of this method. For instance, the difficulty in precisely controlling the cooling rate during each cooling step makes this method commercially impractical when producing steel sheets with various thickness and/or widths, as requested by different customers.
U.S. Pat. No. 4,854,976 to Era et al. provides a method of producing a multi-phase structured cold rolled high tensile steel sheet. The composition of this steel sheet includes, in weight %, 0.08 to 0.25% carbon, 0.3 to 2.0% silicon, 0.6 to 1.8% manganese, 0.04 to 0.20% phosphorus, not more than 0.10% aluminum, and not more than 0.01% boron. The composition is subjected to hot rolling under the condition that the coiling temperature is less than 600° C. and cold rolling. The cold rolled steel is heated for 1 to 10 minutes at a temperature in the range Ac1+10° C. to Ac3-10° C., then quenched at a cooling rate greater than 50° C./s set up to a temperature 350 to 500° C. with a holding period of 1 to 10 minutes at that temperature before final air cooling. The microstructure of the cold rolled annealed steel has ferrite, bainite and retained austenite, with or without a small amount of martensite, and the percentage of the retained austenite being more than 8%.
As disclosed in the U.S. Pat. No. 4,854,976, carbon is added in a high concentration into the steel sheet in order to obtain high hardenability and strength, which can adversely affect the formability and weldability of the steel. In addition, the above patent also employs phosphorus as a strengthening element. When phosphorus is near the upper limit as described in the '976 patent, the segregation of phosphorus at grain boundaries could occur, which results in brittleness of the steel sheet, and in turn impairs its formability and fatigue property. Moreover, the spring back angle of parts formed from the steel sheet could also be increased. In other words, the shape-fixability of the steel sheet becomes worse. Regarding the manufacturing processes, the castability and rollability of the steel sheet are also deteriorated when too much phosphorus is added. Furthermore, a high phosphorus concentration in steel could adversely affect coating adhesion during the hot dip coating processing.
International Patent Publication No. WO 2004/057048 A1 to Antonissen, et al. is related to a steel composition made by a process comprising a cold rolling step for the production of uncoated, electro-galvanized or hot dip galvanized TRIP steel products. The composition is also characterized by a specific addition of phosphorus. The composition includes, in weight %, 0.13 to 0.26% carbon, 1.0 to 2.2% manganese, 0.8 to 1.5% aluminum, 0.2 to 0.6% silicon, 0.04 to 0.1% phosphorus, not more than 0.012% sulfur, not more than 0.02% nitrogen, not more than 0.10% titanium, not more than 0.10% niobium, not more than 0.10% vanadium, and not more than 0.0010% boron.
As disclosed in the Patent Publication No. WO 2004/057048, the steel produced is a TRIP steel. The high concentrations of carbon and aluminum in this steel would significantly deteriorate its coating adhesion, surface quality and weldability. As stated earlier, the difficulty of welding this type of steel has become a significant challenge for the steel in automotive industry, and limits its automotive applications.
Japan Patent Publication No. 2003/342644 to Yoshida Hiromi et al. discloses a process for manufacturing a multi-phase metallographic structure type hot dip galvanized cold rolled steel sheet. The steel sheet has a composition which consists, by mass, of 0.01 to 0.05% carbon, 0.1 to 1.0% silicon, 1.0 to 3.0% manganese, not more than 0.10% phosphorus, not more than 0.02% sulfur, 0.005 to 0.1% aluminum, not more than 0.02% nitrogen, 0.01 to 0.2% vanadium, and 0.001 to 0.2% niobium, where the respective contents of vanadium (V), niobium (Nb) and carbon (C) satisfy a relation of 0.5×C/12≦(V/51+Nb/93)≦2×C/12. Titanium is disclosed in an amount between 0.001% and 0.3% and satisfying a relation of 0.5×C/12≦(V/51+Nb/93+Ti/48)≦2×C/12.
Japan Patent Publication No. 2004/002909 to Yoshida Hiromi et al. provides a process for manufacturing a multi-phase hot dip galvanized cold rolled steel sheet. In the manufacturing process, after cold-rolling, the steel sheet is subjected to a primary continuous annealing and a secondary continuous annealing. The steel slab has a composition comprising, by mass, 0.01 to 0.05% carbon, 0.1 to 1.0% silicon, 1.0 to 3.0% manganese, not more than 0.10% phosphorus, not more than 0.02% sulfur, 0.005 to 0.1% aluminum, not more than 0.02% nitrogen, 0.01 to 0.2% vanadium, 0.005 to 0.2% niobium, provided that the contents of vanadium (V), niobium (Nb) and carbon (C) satisfy a relation: 0.5×C/12≦(V/51+Nb/93)≦2×C/12. Titanium is disclosed in an amount between 0.001% and 0.3% and satisfying a relation of 0.5×C/12≦(V/51+Nb/93+Ti/48)≦2×C/12.
Vanadium is used in the compositions of these two Japanese patent publications in high concentrations. When the concentration of this element is close to or above the middle range of the limit as described in these patents, the vanadium carbides or vanadium nitrides are respectively precipitated out in the steel sheet. Since these types of precipitates are usually formed on grain boundaries, they can not only markedly reduce castability during manufacturing the steel sheet, but also can deteriorate the formability of the steel sheet when forming or press forming the produced steel sheet into the final automotive parts. Moreover, the impact toughness, fracture performance, crashworthiness, stretch formability and stretch flangeability of the steel sheet could also be reduced due to the occurrence of these precipitates.
A hot dip coated, high strength, complex metallographic structured or multi-phase structured steel is presently disclosed that improves fracture performance during stamping and forming, while possessing one or more of the following properties: excellent formability, excellent fracture performance, excellent stretch formability, excellent stretch flangeability, excellent dent resistance, excellent durability, excellent impact performance, excellent intrusion and crash resistance and excellent weldability.
A complex metallographic structured steel sheet is disclosed comprising:                (a) a composition comprising the following elements by weight:                    carbon in a range from about 0.02% to about 0.2%,            manganese in a range from about 0.2% to about 3.5%,            phosphorous less than or equal to about 0.1%,            sulfur less than or equal to about 0.03%,            silicon less than or equal to about 1.2%,            aluminum in a range from about 0.01% to about 0.10%,            nitrogen less than or equal to about 0.02%,            copper less than or equal to about 0.8%,            vanadium less than or equal to about 0.12%,            one chosen from molybdenum, chromium, nickel, and a combination thereof, in a range between about 0.05% and about 3.5%, and            one chosen from titanium, niobium and a combination thereof, in a range between about 0.005% and about 0.8%, wherein, if present, titanium (Ti) is present with nitrogen (N) and sulfur (S) satisfying a relationship Ti* greater than or equal to about 0.01% and less than or equal to about 0.6%, where Ti* equals (Ti−(24/7)N−(3/2)S),            and the balance of the composition comprising iron and incidental ingredients;                        (b) a multi-phase microstructure having in combination ferrite, martensite between 3% and 65% by volume, and at least one microstructure selected from the group consisting of bainite and retained austenite, and having fine complex precipitates selected from the group of TiC, NbC, TiN, NbN, (Ti.Nb)C, (Ti.Nb)N, and (Ti.Nb)(C.N) particles having at least 50% smaller than 20 nm in size, and        (c) physical properties comprising tensile strength greater than about 780 megapascals and at least one of the following properties of elongation greater than about 10%, yield ratio greater than about 70%, and hole expansion ratio greater than about 50%.        
Alternately, the martensite phase of the microstructure may be between 10% and 35% by volume. The bainite phase of the microstructure may be between about 2% and about 20% by volume of the microstructure, or alternately may be between about 5% and about 15% by volume. The retained austenite phase of the microstructure may be between about 1% and about 12% by volume of the microstructure, or alternately may be between about 3% and about 8% by volume. The ferrite in the microstructure may be between 20 and 85% by volume of the microstructure. Further, the complex metallographic structured steel may have a hot-dipped coating chosen from zinc, aluminum, or an alloy thereof.
The composition may contain a purposeful addition of calcium less than or equal to about 0.02%.
The complex metallographic structured steel may have a yield strength at least about 650 megapascals, and may have an impact strength greater than about 1200 gram-meters measured on a Charpy V-notch specimen 1.5 millimeters thick. The complex metallographic structured steel may have weldability characteristic defined by a weld current range greater than 2 kiloamperes measured for a weld time greater than 15 cycles in a 1.5 millimeter thick sheet that is galvanized and not galvannealed.
Presently disclosed is a practical manufacturing method of reliably making the complex metallographic structured or multi-phase structured steel, which may be carried out by steel manufacturers with little or no increase in manufacturing cost.
A method of making a complex metallographic structured steel sheet may comprise                a) assembling a continuous metal slab caster having a casting mold,        b) introducing molten steel into the casting mold and continuously casting a molten steel into a slab having a composition comprising the following elements by weight:                    carbon in a range from about 0.02% to about 0.2%,            manganese in a range from about 0.2% to about 3.5%,            phosphorous less than or equal to about 0.1%,            sulfur less than or equal to about 0.03%,            silicon less than or equal to about 1.2%,            aluminum in a range from about 0.01% to about 0.10%,            nitrogen less than or equal to about 0.02%,            copper less than or equal to about 0.8%,            vanadium less than or equal to about 0.12%,            one chosen from molybdenum, chromium, nickel, and a combination thereof, in a range between about 0.05% and about 3.5%, and            one chosen from titanium, niobium, and a combination thereof, in a range between about 0.005% and about 0.8%, wherein, if present, titanium (Ti) is present with nitrogen (N) and sulfur (S) satisfying a relationship Ti* is greater than or equal to about 0.01% and less than or equal to about 0.6%, where Ti* equals (Ti−(24/7)N−(3/2)S),            and the balance of the composition comprising iron and incidental ingredients;                        c) hot rolling the steel slab having an exit temperature in a range between about (Ar3−60)° C. and about 1000° C. (about 1832° F.);        d) cooling the hot rolled steel at a mean cooling rate of at least about 3° C./s (about 5.4° F./s);        e) optionally, coiling the steel at a temperature between about 400° C. (about 752° F.) and about 800° C. (about 1472° F.);        f) cold rolling the steel to a desired steel sheet thickness, with the cold rolling reduction being at least about 25%;        g) heating the steel sheet to a temperature in the range between about 625° C. (about 1157° F.) and about 925° C. (about 1697° F.) for between about 10 seconds and 10 minutes; and        h) cooling the steel sheet to a temperature in the range between about 400° C. (about 752° F.) and about 550° C. (about 1022° F.) at a cooling rate of at least 3° C./s to obtain a multi-phase microstructure having in combination ferrite, martensite between 3% and 65% by volume, at least one microstructure selected from the group consisting of bainite and retained austenite, and having fine complex precipitates selected from the group of TiC, NbC, TiN, NbN, (Ti.Nb)C, (Ti.Nb)N, and (Ti.Nb)(C.N) particles having at least 50% smaller than 20 nm in size, and physical properties comprising tensile strength greater than about 780 megapascals and at least one of the properties of elongation greater than about 10%, yield ratio greater than about 70%, and hole expansion ratio greater than about 50%.        
Optionally, the cold rolling reduction may be at least about 35%. Further, the method of making a complex metallographic structured steel may include the steps of dipping the steel sheet through a bath of coating material to coat the surface of the steel sheet with the coating; and further cooling the sheet to a desired temperature. The hot dip coating may be annealed at a temperature in a range between about 450° C. (842° F.) to 650° C. (1202° F.). The cooling in step (h) may be between about 3° C./s and 25° C./s.
Again, the composition may contain a purposeful addition of calcium less than or equal to about 0.02%.
The invention is now discussed in connection with the accompanying Figures and the Examples described below.